I actually think that looks is where the S2 lets itself down. Yes it's got an OLED screen but other than that, it's a bit too light, doesn't feel like it'd survive a few bumps and a little too plastic fantastic for my liking.
I think the UI is a bit mickey mouse when compared to Sense UI on the Sensation.
But the reason I'd go for the iphone over the S2 is quite simply because with Apple you get the whole shebang. Yes yes, I know ppl will go ah yes but no flash....and i don't like iTunes blah blah. Yet for 99% of people it's not a big deal.
It's a great looking phone, everything about it oozes quality. The app market is mature. The apps are also way more polished than some of the absolute rubbish on the Android market.
Being the most popular phone out you can get screen protectors, cases, accessories practically anywhere. Even in Tesco's! The range is unbelievable. Buy a new car and chances are it will have ipod connectivity. Sync it with bluetooth and it will drag across your phonebook. Get a new stereo and it'll probably have an ipod dock.
In short, it...just....works. Leaving you more time to do practical stuff rather than fiddle with it for fiddling's sake.
Lastly Apple customer service is without a doubt the best around. If there are problems, take it into their store (doesn't matter where you bought it) and they'll sort it out there & then for you.
Don't get me wrong, the S2 is undoubtedly a great phone. But in my experience if you are getting a two year contract, you want the best you can afford and having the best hardware is only part of the equation. Remember, 2 years is quite a hell of a commitment. I've had plenty of girlfriends didn't last that long! 